1. Field of the Invention
The present invention relates generally to an apparatus to be used in a two-sided contact printing station and particularly as directed to improvements for obtaining a high precision alignment between two plates on opposite sides of a material to be printed.
2. Description of Prior Art
In the present systems for two-sided contact printing, a typical arrangement includes a thin sheet of light-sensitized working material inserted between a pair of photographic plates attached to suitable frames. The photographic plates each contain a pattern which is projected onto a respective light-sensitized surface of the working material using well known photographic techniques.
Some difficulties have been encountered with the plate mountings used heretofore. There has been a tendency for the frames to shift during normal use so that frequent adjustment has been required to maintain precise registration of the plates with respect to each other. Therefore, in addition to the pattern to be photographically reproduced on the working material, the plates typically carry an alignment system to insure precise registration of the projected patterns. The alignment system may take the form of imprinted registration blocks or indices on each of the plates which must be aligned with one another. For example, U.S. Pat. No. 3,199,430 describes an alignment system where a visual indication of alignment is provided by registration blocks on the plates, and a plurality of adjusting levers are provided to manipulate the plates into position. This lever adjustment has proven too tedious and time consuming for use in a high production environment and has typically been replaced by very complex and costly automated electro-mechanical controls to provide consistent results.
FIGS. 2 and 3 illustrate another known alignment system which includes attaching a number of fixed profile alignment pins 20 or 20' to one of the plates 15, and providing matching female sockets or bushings 21 or 21' in the other plate 16. The system of FIG. 2 utilizes a tapered pin 20 and a corresponding tapered socket or bushing 21. The system of FIG. 3 utilizes a straight diameter pin 20' and a corresponding straight diameter socket or bushing 21'. While providing a low-cost alternative to the automated visual systems, the fixed profile alignment pin systems often suffer from an inherently low precision caused by worn out contact surfaces after prolonged use. Even a small degree of wear is unacceptable in many applications, such as the manufacture of CRT aperture masks and grills, where a precision range of +/-0.0002 inches may be required.